If your not sending off your lambs, and willing to pay a little more Clik is a great product, ok so long withdrawal, but put that on and you ll be safe in the knowledge that it will do the job well. It's on the likes of mole valley, don't think it comes in small bottle like crovect does with they're range.
Battles maggot oil I hear is good for treatment too, again NOT a preventative and much much cheaper than treating with spot on. Maggot oil well under twenty quid for a litre. Spot on 60 odd maybe can't remember for the smallest bottle, couple of hundred ML.
We have only had problem were we had to do some lambs with crovect and just gone over cover, a handful, majority been struck above tail.all healing well and bright as buttons.
We use crovect to kill them and AB spray, never purple.